1. Field of the Invention
This invention relates to a method and system for GPS position measuring and a frequency error detecting method.
2. Description of Related Art
In a receiver using satellites for position measuring (GPS for example), pseudorandom codes received from the satellite are detected and synchronized (i.e., aligned with) with the same codes generated within the receiver.
However, because the satellites are moving, the signals have a Doppler shift and the frequency of a received signal is different from the frequency transmitted from the satellite. In order to detect the received signal, the receiver must search not only for the proper code alignment, but also must locate the Doppler-shifted frequency of the received signal and compensate for the Doppler shift so that correlation processing can extract the signal from the noise.
When the internal oscillator in the receiver has an unknown frequency error, this error is added to the Doppler shift of the signal caused by satellite motion.
In order for the receiver to compensate for the Doppler shift by itself, it must look for the received signal at many frequencies which span the expected range of Doppler shift plus the range of error in the receiver local oscillator frequency. A large amount of correlation processing is required at each frequency in this range to make the signal visible. Thus, there is a significant disadvantage in that considerable time is consumed until the receiver can synchronize its pseudorandom code and compensate for the Doppler shift on the signal.
Even if the approximate Doppler shift of the satellites is provided by an outside source, the frequency uncertainty in the receiver's local oscillator can still mandate an extensive search for the received signal over a large frequency range. A very accurate oscillator can solve this problem. However, only an expensive oscillator, such as a rubidium oscillator, can provide the required frequency accuracy.
To overcome the disadvantage of a time-consuming frequency search, the standard practice is to synchronize the receiver's local oscillator with received signals having an accurate frequency. This is typically done by locking the phase of the receiver's local oscillator to that of the received accurate frequency signal (such as is done in clocks which receive signals from the atomic standard radio station JJY). This means is generally known as a phase-locked loop (PLL) circuit (refer to the Japanese Patent Provisional Publication No. H7-321644, for example).
This conventional method has the disadvantage that detecting the error of the receiver's local oscillator by reference to an outside signal is extremely difficult when the outside signal is influenced by Doppler shift or is embedded in noise.
It is therefore the object of the present invention to provide a GPS position measuring method and system, and a frequency error detecting method in which the signal-to-noise (SNR) ratio of a very weak signal covered by noise is significantly improved, the frequency error in the receiver's local oscillator can be self-detected, and the processing time of the detection can be made short.